WebBrowser is most likely the wrong tool for this task. You would use WebBrowser when you want to actually be able to see the web page on the screen. It looks like you are instead just trying to POST something and get the result.
Try WebClient
instead, it is much simplier and works much better for POST:
using (var client = new WebClient())
{
client.Headers["User-Agent"] = "Mozilla/4.0 (Compatible; Windows NT 5.1;
MSIE 6.0) (compatible; MSIE 6.0; Windows NT 5.1;
.NET CLR 1.1.4322; .NET CLR 2.0.50727)";
const string url = "http://www.example.com";
var inputs = new NameValueCollection {
{"search_div_id", search_term},
}
var response = client.UploadValues(url, inputs);
string html = Encoding.UTF8.GetString(response);
}